This application claims the priority of German Application 102 07 986.2 filed Feb. 25, 2002, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust-gas purification system for an internal combustion engine, which, as seen in the direction of flow of the exhaust gas, has a particle filter, provision for a reducing agent in the exhaust-gas flow, a catalytic converter for reducing nitrogen oxide which is present in the exhaust gas, and a downstream oxidation catalytic converter.
DE 40 32 085 A1 has disclosed a catalytic converter arrangement for the reduction of nitrogen oxides. In that known catalytic converter arrangement, a soot particle filter, a temperature peak buffer, an injection device for ammonia, a flow mixer, a first denox catalyst bed, a second denox catalyst bed and an oxidation catalyst bed are arranged in a common housing, one behind the other as seen in the direction of flow. The first denox catalyst bed has its optimum efficiency at higher temperatures than the second denox catalyst bed.
EP 0 806 553 A2 has disclosed a method for exhaust-gas purification in diesel engines, in which fuel is added to the exhaust-gas passage in order to wet soot in a soot filter and to enrich the levels of HC. An oxidation catalytic converter may be provided upstream of the soot filter, and an oxidation catalytic converter or an SCR (Selective Catalytic Reduction) catalytic converter with integrated oxidation catalytic converter may be provided downstream of the soot filter. To enrich the levels of HC in the soot in the soot filter, a liquid fuel is added to the exhaust-gas passage.
EP 0 896 831 A1 has disclosed an exhaust-gas purification system for catalytic reduction of nitrogen oxides in which urea is introduced into the exhaust-gas path as a reducing agent. The urea is introduced upstream of a combination component comprising evaporator, gas mixer and hydrolysis catalytic converter. An oxidation catalytic converter may be provided upstream of the combination component and the injection device. A reduction catalytic converter may be provided downstream of the combination component and may be followed, as seen in the direction of flow, by an ammonia (NH3) barrier catalytic converter.